1. Field of the Invention
The present invention relates generally to cooling systems for battery and electronic component enclosures. More specifically, the present invention relates to a fan controller for a fan-based heat exchanger for use in outdoor cabinets that house electronic components such as telephone digital loop carrier equipment and data and video transport equipment.
2. Background of the Invention
Telephone companies typically deploy outdoor cabinets to house electronic telephone equipment such as ADSL multiplexors. The cabinets typically have two chambers. One chamber houses the electronics. The second chamber houses a power source such as a battery. An exemplary cabinet is the Lucent 52B cabinet.
The outdoor cabinets typically include a fan-based heat exchanger to provide cooling to the electronics and battery as well as to remove gases that may build up due to the operation of the battery. Typically, the heat exchanger is driven by fans located in the door or heat exchanger of the cabinet. The fans create airflow that draws air up through the door or heat exchanger of the cabinet to the outside. This airflow extracts heat from the internal chambers of the cabinet.
Operation of the fans is controlled by a fan controller. In a conventional heat exchanger, the fan controller causes the fans to be either entirely “on” or entirely “off.” FIG. 1 is a schematic diagram of a prior art fan controller 100. Fan controller 100 controls a fan 102a and a fan 102b. A power source 104 provides power to operate fans 102a and 102b when a thermostat 106 is closed. Conventionally, thermostat 106 is set to operate at, for example, approximately 13 degrees Celsius. That is, when the ambient temperature within the cabinet or within the battery compartment is greater or equal to 13 degrees Celsius, thermostat 106 closes. Closure of thermostat 106 supplies power to fans 102a and 102b to operate them. Once operating, the fans usually do not shut off until the ambient temperature falls below, for example, approximately 10 degrees Celsius. Consequently, in summer months, the fans are likely to be on all the time. Even in other months, the fans are likely to be on almost 90 percent of the time.
One problem with conventional heat exchangers is that the two fans generally operate at least 90 percent of the time, even when maximum cooling is not required. This long, almost continuous, operation of the fans significantly reduces the expected lifetime of the fans. With large numbers of deployed cabinets, the requirement to send service personnel to replace failed fans is very expensive. The costs are not limited to the cost of the fans themselves, but also include the cost of the service personnel and fleets of vehicles to transport the service personnel.
Another problem associated with conventional cabinets is that air intakes (where the air is sucked in from the outside) are typically only about 4 inches off the ground. This poses no problem in the relatively clean environment of a manufacturer's testing lab. In the outdoor environment in which the cabinets are typically deployed, however, the low placement of the intakes tends to allow the fans to suck in material or debris such as grass clippings, seeds, dirt and dust into the interior of the cabinet. Because the fans operate for such long periods, a lot of material and debris can be sucked up through the intakes. This material and debris reduces the efficiency of the heat exchanger. The reduction in efficiency of the heat exchanger adversely affects the operation of the electronics and the battery as well as significantly reduces their operational lifetimes.
Another problem is due to a fine mesh screen that covers the air intakes. The mesh screen prevents rodents and insects from getting into to the cabinet. However, over a period of time, the screens often become clogged. The clogging obstructs airflow through the cabinet. This creates a vacuum effect in the interior chambers of the cabinet. As a result, when it rains, water can be sucked up into the cabinet through any crack or crevices in the exterior shell of the cabinet. The suction of the fans prevents the water from leaking out. The trapped water can have a corrosive effect on the electronics and battery inside the cabinet. This lowers the life expectancy of the battery and reduces the reliability of the electronics.
One method that has been tried to solve the foregoing problems was to reverse the direction of the fans, so that the fans blow air out of the cabinet. This solution had the intended effect of reducing clogging of the intake meshing. However, this solution also had the unintended effect of increasing the internal temperature of the cabinet to a point above a safe operating temperature for the battery or electronics. Consequently, this technique could not be used in most cases.